If the QR Code you have made resolves to a url make sure the page is optimized for display on a mobile device, that is the first rule of QR Codes!
It is easy to forget that QR Codes are primarily for mobile devices and that they should resolve to a mobilized version of the encoded url.
For a few well known sites it will not matter because they will examine the user agent (a string identifying a browser or agent accessing a web page) and display the appropriate version of the page. For example if you access http://www.youtube.com/ from your PC using IE7 your user agent may look something like this Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) and you will see the normal page. However if you are using an iPhone your user agent will look something like this Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) and you will be redirected to http://m.youtube.com/ the mobile version of YouTube.
Most QR Codes you generate will be for sites that do not have user agent switching but fortunately you can still ensure that the mobile user of your QR Code sees an optimized version.
If the site is not under your control then you can use Google’s mobilizer by adding the url to this string:
For example to mobilize the MSN home page:
And that would be the url to use with your QR Code generator.
If you are mobilizing your own site such as your blog then there is an even better option which will mobilize your site, generate a QR Code for the mobilized url and keep usage statistics, it’s called Delivr. Another possibility to consider is Mippin which has an option to include advertising but you must have an RSS feed for it to work. It is being used on this site at the moment (without the advertising!), click on the mobile link (top right) to see the results.
Whatever you end up doing make sure the user of your QR Code sees content optimized for mobile devices because 99.9% of the time they will be using a camera phone.